S. Ohira

Cu-site-substitution effects on the 1/8 anomaly in the high-Tc cuprates and on the anomaly at x=0.21 in La2−xSrxCuO4

Effects of the Cu-site substitution on the 1/8 anomaly in the Bi-2212 and Y-123 phases and also on the anomaly at x = 0.21 in La 2-x Sr x CuO 4 (LSCO) have been investigated from the transport, muon-spin relaxation and neutron-scattering measurements. It is suggested that the dynamical stripe correlations of spins and holes exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by Zn at the hole concentration of ∼1/8 per Cu in the high-T c cuprates and also at x = 0.21 in LSCO. Based upon the stripe-pinning model, universal properties in the high-T c cuprates that partially substituted Zn is a strong scatterer and that the Zn substitution markedly suppresses the superconductivity are also discussed.

Anomalous magnetic field dependence of muon spin relaxation in double-chain system KCuCl3 and TlCuCl3

Abstract In μ+SR studies on the S= 1 2 double-chain systems KCuCl3 and TlCuCl3, which are thought to have a spin singlet ground state with finite spin gaps, we found that the muon spin becomes strongly depolarized below 20 and 10 K, respectively. We observed an increase in the relaxation rate with increasing longitudinal magnetic fields at 4.8 K in KCuCl3. At 1.8 K, we observed no decrease of the relaxation rate with increasing longitudinal fields. However, the shapes of these spectra cannot be represented by using usual formulae. Such an anomalous behavior was not seen in TlCuCl3. We discuss the origin of the muon spin relaxation and difference between the properties of KCuCl3 and TlCuCl3.

Ab initio molecular orbital studies of the positive muon and muonium in 4-arylmethyleneamino-TEMPO derivatives

Abstract The muon and muonium bonding sites of the 4-arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical crystals with aryl groups consisting of biphenyl and 4-pyridyl were studied via ab initio Hartree–Fock theory. The hyperfine fields, including both intramolecular and intermolecular interactions, were calculated at the sites of interest and compared to zero field μSR results.

Possible stripe ordering at x = 0.21 in La2-xSrxCu1-yZnyO4

Abstract Anomalies, which we have recently discovered at x =0.21 in La 2− x Sr x Cu 1− y Zn y O 4 single crystals, are reviewed. In the 1% Zn-substituted crystal with x =0.21, the resistivity exhibits an upturn at low temperatures below about 80 K and the superconductivity (SC) is markedly suppressed. The muon-spin-relaxation ( μ SR) measurements have revealed the existence of a static ordered state of Cu spins in this crystal at low temperatures below 2 K. From the elastic neutron-scattering experiment, incommensurate magnetic peaks around ( π , π ) have been observed in this crystal at low temperatures below 20 K. Even in the Zn-free crystal with x =0.21, the SC is a little suppressed and the magnetic correlation has been found from the μ SR measurements to be enhanced at low temperatures below about 0.8 K. These results strongly suggest that the dynamical stripe correlations of spins and holes tend to become static singularly at x =0.21 and are pinned by a small amount of Zn at x =0.21, leading to the static stripe-order and the marked suppression of SC, as in the case of x =0.115.

Radio-frequency muon spin resonance of Haldane-gap systems, Ni(C2H8N2)2NO2(ClO4) and NiC2O42·(2-methylimidazole)

In quantum spin systems with a singlet ground state, an enhancement of the muon spin relaxation was frequently observed at low temperatures, though the ground state is non-magnetic. In order to clarify the origin of this enhancement, we have measured the muon spin resonance with radio-frequency field (RF-μSR) for Haldane-gap systems, Ni(C2H8N2)2NO2(ClO4) (NENP) and NiC2O42·(2-methylimidazole). Possible origins of an enhancement of the muon spin relaxation at low temperatures are discussed.

μSR study of a mixed compound NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x

We examined the muon spin relaxation (μSR) of mixed compounds NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x with x=1.0 and 0.49. Although the macroscopic magnetic properties are obviously different from each other, both systems exhibit similar behavior in the muon spin relaxation. In addition, in the x=0.49 (SG) sample, a critical slowing down of spin dynamics was not observed in this μSR measurement, though the spin-glass like freezing was observed in the susceptibility measurements. Qualitative explanation of these anomalous observations is given.

MuSR/μ+SR studies in KDP and DKDP

The temperature dependence of muon interactions has been studied in ferroelectric KDP ( H2KPO4) and DKDP ( D2KPO4) using conventional μSR and muon spin resonance spectroscopy. In longitudinal field measurements, a fast relaxing component and a slow relaxing component were observed. The slow relaxing component is attributed to diamagnetic muons. The muon spin resonance measurements indicate that the fast relaxing component results from some muonium like species: either normal or anomalous. In zero field and weak longitudinal field μSR (0–100 G), a remarkable peak in the fast relaxing component is observed around 220 K in both KDP and DKDP. An additional feature is also seen around 300 K. The amplitude of the resonance measurement has a broad minimum around 200 K which corresponds to the maximum in the relaxation rate in longitudinal field (100 G). The temperature dependence of the muonium relaxation rate in KDP is almost identical to that of DKDP. The diamagnetic fraction also shows almost no difference in relaxation rate or asymmetry for DKDP and KDP.

μSR study on a metal complex having a ferromagnetic linear chain p-NPNN·Cu(hfac)2

Abstract The magnetic properties of p-NPNN·Cu(hfac)2 (p-NPNN= p-nitrophenyl nitronylnitroxide, hfac= hexafluoroacetylacetonat) were studied by muon spin relaxation (μSR). From an AC susceptibility measurement, the system is suggested to exhibit an antiferromagnetic ordering at 1.22 K and a spin canting at ∼0.6 K . We observed a drastic change of the μSR time spectra between 1.2 and 1.1 K . The initial asymmetry suddenly decreased due to large internal fields and a part of the muon spin polarization did not relax below the magnetic phase-transition temperature, indicating the appearance of static internal magnetic fields. No remarkable change associated with the spin canting at 0.6 K was observed.

Theory of nuclear quadrupole interactions of 14N, 17O, and 35Cl nuclei in p-Cl-Ph-CH-N=TEMPO

The nuclear quadrupole coupling constants and asymmetry parameters have been studied for the 35Cl, 17O, and 14N nuclei in the molecular ferromagnet 4-(p-chlorobenzylideneamino)-TEMPO (2,2,6,6-tetramethyl- piperidin-1-yloxyl) using elctronic stuctures obtained by the Hartree-Fock procedure for the bare system and systems with trapped muon and muonium. Trends in the sizes of the coupling constants and asymmetry parameters for the various nuclei have been studied, and possible physical explanations have been proposed. For the systems with trapped muon or muonium, very substantial influences of the muon and muonium on the coupling constants and asymmetry parameters for the nuclei close to the trapping sites have been observed. The coupling constants and asymmetry parameters are found to be very different for the various nuclei, for the two cases where muon is trapped near chlorine and muonium near oxygen, indicating that, if experimental data were available to compare with theory, one could make conclusions about which of these two centers is responsible for the observed muon spin rotation frequency associated with the muon magnetic hyperfine interactions in these two trapped systems

Theoretical investigation of muon and muonium trapping and hyperfine interactions in the chemical ferromagnet p-NPNN (β-phase)

The trapping sites for muon and muonium in β-phase ferromagnetic p-NPNN have been determined by the first-principles Unrestricted Hartree–Fock procedure. Four trapping sites are found for the muon near the two nitrogen and two oxygen atoms of the two NO groups. For the singlet state of trapped muonium, two trapping sites are found near the two oxygens of two NO groups and for the triplet state two trapping sites are found near the two oxygens of the NO2 group. The observed μSR signal at zero field with frequency 2.1 MHz is assigned to the singlet muonium sites near the two oxygens of the two NO groups and the high frequency signal ascribed to an isotropic hyperfine constant of 400 MHz is assigned to the two trapped muon sites near the two nitrogen atoms of the two NO groups.